Investigation into the Wakatipu Basin Aquifers

Abstract

Background: The Otago Region Council (ORC) is responsible for managing Otago’s water resources on behalf of the Crown and the community. The basin investigations that ORC conducts from time to time for the 20-plus groundwater areas around Otago are important building blocks in delivering informed water resource management. In recognition of population increase and land-use change in the Wakatipu Basin in the last 20 years, ORC saw the need for a fresh groundwater-resource investigation into its aquifers, including the Shotover floodplain and Frankton Terrace. Why is allocation of groundwater necessary? Compared to surface water, groundwater resources are not visible. Instances of the overexploitation of groundwater elsewhere have caught communities by surprise, partly due to this lack of visibility. Signs of over-exploitation are hard to spot and occur gradually, often over decades. Placing a limit on the amount of groundwater that can be taken in any particular year, and implementing other controls, helps to prevent the resource from being outstripped. Why is water quality management needed? Substances that can affect groundwater quality tend not to be visible, either. Often the only signs that groundwater quality is becoming affected by contaminants are long-term trends observed in regular analysis of bore water. Once water in an aquifer becomes contaminated, it can take years or decades for the contaminants to be flushed out. It is important, therefore, to look periodically for any discernible trends and to forecast the potential of contamination using scientific techniques. What has this study found? The Wakatipu Basin aquifers were found to be scattered areas of glacial-gravel deposits, separated by schist ridges and major bedrock hills, such as Slope Hill and Morven Hill. The main groundwater system associated with the Mill Creek/Hayes Creek drainage was found to be further subdivided by the waterfall at Millbrook Resort and Lake Hayes. Outlying aquifers were also found at Hawthorn and Morven Ferry. In addition, the Shotover River has lain down gravel deposits to either side of its channel between Arthurs Point and the Kawarau River confluence that contain an alluvial aquifer. The Frankton Terrace has an underlying aquifer of considerable depth and substantial groundwater volume, with the water table lying at Lake Wakatipu level. The various aquifers throughout the Wakatipu Basin are mainly of value for providing domestic water to public, communal and individual water supplies, with very little high volume abstraction being used for irrigation or industry. The aquifers are replenished by rainfall, rivers, creeks, feed springs and out flowing seepage into the basin. What should be done next? At a regulatory level, the value of the Wakatipu Basin aquifers for providing drinking water should be recognised as ‘groundwater protection zones’ (GPZs). This report proposes that maximum-abstraction limits be taken to the public and water users for consultation and ultimate adoption.